The present embodiments relate to a method for determining a four-dimensional (4D) plan for carrying out intensity-modulated radiation therapy of a target volume with irregular periodic motion, with a radiation therapy apparatus.
Methods for determining 4D plans for Intensity-Modulated Radiation Therapy (IMRT; including static IMRT, dynamic IMRT, arc IMRT (IMAT) and hybrid IMRT) are known in the prior art.
In a 4D plan, the 3D radiation plans contained in the 4D plan are optimized in each case for the assigned motion phase. It is assumed in such cases that the target volume moves while the radiation therapy is being carried out in a manner, on which the planning is based.
In reality, however, discrepancies occur between the motion of the target volume underlying the planning and the motion of the target volume while radiation therapy is being carried out. Thus, for example, the motion of a lung tumor (e.g., a target volume) depends on the breathing motion of the lungs. If the breathing motion of a patient on whom the 4D plan is based differs from the breathing motion of the patient when the radiation therapy is being carried out, then radiation therapy carried out in accordance with the 4D plan does not lead to the desired radiation therapy results. The 4D plans may therefore be robust in relation to motion discrepancies of the target volume from the motion underlying the 4D plan.
The collimators used for beam forming (e.g., multi-leaf collimators (MLC)) are subject to restrictions during a change from one aperture setting to a next aperture setting. Thus, for example, the leaves of a multi-leaf collimator are moveable at a given maximum speed, so that a change from an aperture setting to a next aperture setting uses a minimum time. The 4D plans may, therefore, also be determined to take account of the restrictions of the collimator.
In order to solve these problems, the article “Motion management with phase-adapted 4D-optimization,” by Omid Nohadani et al., in Phys. Med. Biol. 55 (2010) 5189-5202 proposes a method for determining 4D radiation therapy plans that is designed to be robust in relation to irregular motion discrepancies. The method is based on determining fluence maps optimized towards the therapy objective. The method is restricted by fluence maps for adjacent motion phases. On the basis of the fluence map aperture settings of a collimator with adjustable aperture (e.g., MLC) are then determined for the 4D plan. It is assumed that similar fluence maps, when converted into aperture settings, create similar apertures. The 4D plan thus defines a plurality of positions of the radiation source aperture settings of the collimator assigned to these positions, which are produced on the basis of optimized fluence maps.
On the subject of “optimization of fluence maps,” “Fluence Map Optimization in IMRT Cancer Treatment Planning and A Geometric Approach,” by Yin Zhang and Michael Merrit, July 2004, revised May 2005, National Cancer Institute, National Science Foundation, (http://www.caam.rice.edu/˜zhang/reports/tr0412.pdf) is provided.